When the desired grooves or holes are to be patterned on a semiconductor wafer or the like, an etching process is used wherein the top surface of the substrate (the top surface where etching is not going to be carried out) is covered with a mask (photoresist layer) to make a mask surface, leaving out the surface to be etched (the portion which will become a groove or hole), plasma is generated using etching gas, and etching is performed on the surface to be etched. In the etching process, it is necessary to monitor, in real time, the etching depth which changes over time, in order to process the surface to be etched to the desired depth.
In monitoring of etching depth, a method is employed which utilizes the interference of light due to difference in depth between the region covered by the mask and the region of the surface to be etched. The method most often used among monitoring methods which employ optical interference is a method whereby change over time of interference intensity accompanying change in etching depth is monitored by projecting a single-wavelength light, and the etching depth is measured by counting the waves with maxima and minima of interference intensity (see Patent Literatures 1 through 4). A monitoring method utilizing change over time of interference intensity accompanying change in etching depth indicated by single-wavelength light will be referred to hereinafter as a “change-over-time monitoring method.”
Furthermore, as another monitoring method utilizing interference light, a method has been disclosed wherein light having a certain wavelength bandwidth (wavelength band) is projected, and the reflected interference spectrum is utilized. For example, a method has been disclosed wherein a model predicted spectrum (theoretical value), predicted from a reflection model of a wafer surface film, is computed in advance, and monitoring is performed through spectrum fitting (curve fitting) with actual measurement results of the reflected interference light spectrum (Patent Literature 5). A monitoring method which utilizes spectral interference indicated by a spectrum having a certain wavelength bandwidth will be referred to hereinafter as a “spectral interference method.”